Semiconductor-grade high purity polycrystalline silicon is generally produced by a CVD method referred to as the “Siemens method” by using a chlorosilane gas mainly comprising trichlorosilane as a raw material in the presence of hydrogen. Therefore, it is required that chlorosilanes used as a raw material of high purity polycrystalline silicon should also have an extremely high purity.
In particular, when the impurities contained in raw material chlorosilanes are impurities such as phosphorus and arsenic which serve as a donor in a silicon crystal or impurities such as boron and aluminum used as an acceptor therein, even a very small amount of these impurities will result in having significant influence on the electrical properties (resistivity) of polycrystalline silicon produced. Therefore, practically, it will be considerably meaningful to provide a technique of efficiently removing donor impurities and acceptor impurities contained in raw material chlorosilanes to thereby highly purify the raw material chlorosilanes.
Generally, chlorosilanes for producing polycrystalline silicon are produced by first obtaining a chlorosilane distillate from a metallurgical-grade silicon (so-called metal-grade silicon, hereinafter referred to as “metal silicon”) which contains a relatively large amount of impurities by a publicly known method and then purifying the chlorosilane distillate by a method such as distillation to further highly purify the same.
However, the donor impurities and acceptor impurities as described above are generally contained in metal silicon in an amount in the order of several hundred ppb (atomic) to several hundred ppm (atomic) in terms of the atomic ratio. Therefore, these impurities are not sufficiently removed in the processes of purifying the chlorosilane distillate, but the donor impurities and acceptor impurities may remain in the chlorosilanes finally obtained, which may pose a problem in that such residual impurities may reduce the quality of polycrystalline silicon.
As a method for obtaining a chlorosilane distillate, there is known a hydrogenation step of reacting a tetrachlorosilane (SiCl4)-containing material with hydrogen in the presence of metal silicon to obtain a chlorosilane distillate containing trichlorosilane (SiHCl3) (for example, refer to National Publication of International Patent Application No. 2008-532907 (Patent Literature 1), Japanese Patent Laid-Open No. 58-217422 (Patent Literature 2), Japanese Patent Laid-Open No. 58-161915 (Patent Literature 3), and the like).
This hydrogenation reaction proceeds according to the following reaction formula.3SiCl4+2H2+Si→4SiHCl3  [Formula 1]
The chlorosilane distillate is a fraction of crude chlorosilanes which are the products synthesized by the hydrogenation reaction, and is generally a mixture mainly comprising chlorosilanes such as dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), and tetrachlorosilane (SiCl4).
As another method for obtaining the chlorosilane distillate, there is also known a chlorination step of performing a chlorination reaction by bringing metal silicon into contact with hydrogen chloride in the presence of a catalyst to obtain a chlorosilane distillate containing trichlorosilane (for example, refer to Japanese Patent Laid-Open No. 2005-67979 (Patent Literature 4)).
This chlorination reaction proceeds according to the following reaction formula.Si+3HCl→SiHCl3+H2  [Formula 2]
The chlorosilane distillate is a fraction of crude chlorosilanes which are the products synthesized by the chlorination reaction, and also in this case, the chlorosilane distillate is generally a mixture mainly comprising chlorosilanes such as dichlorosilane, trichlorosilane, and tetrachlorosilane.
The donor impurities and acceptor impurities contained in metal silicon are hydrogenated or chlorinated at the same time when crude chlorosilanes are produced, and are probably mixed into the crude chlorosilanes as a form of compounds having various structures. Such crude chlorosilanes are purified to obtain high purity chlorosilanes, but it is difficult to separate and remove these impurities by a common distillation method when the boiling points of the compounds of the donor impurities and acceptor impurities are close to the boiling point of trichlorosilane.
If polycrystalline silicon is produced by using chlorosilanes from which the donor impurities and acceptor impurities have been insufficiently removed as a raw material, the result will be that a polycrystalline silicon having desired properties cannot be obtained.
Under these circumstances, various methods have been proposed as a method for removing the donor impurities and acceptor impurities in the chlorosilane distillate. For example, there has been proposed a method of adding organic matter to a chlorosilane distillate to produce an adduct with donor impurities and acceptor impurities, followed by distillation and purification to obtain high purity chlorosilanes.
Specifically, Japanese Patent Laid-Open No. 2005-67979 (Patent Literature 4) discloses a method of adding an ether to chlorosilanes, followed by distillation and purification. Further, U.S. Pat. No. 3,126,248 (Patent Literature 5) discloses a method of adding an organic compound comprising dioxane, benzaldehyde, methyl ethyl ketone, dimethylglyoxime, and valerolactone to remove impurities. Furthermore, Japanese Patent Laid-Open No. 2009-62213 (Patent Literature 6) discloses a method of reacting chlorosilanes with oxygen in the presence of benzaldehyde to convert impurities to a high-boiling-point compound and distilling the chlorosilanes after the treatment to separate the high-boiling-point compound of the impurities from the chlorosilanes.
There has been also proposed a method of adding a metal chloride to a chlorosilane distillate to produce an adduct with donor impurities and acceptor impurities, followed by distillation and purification to obtain high purity chlorosilanes.
Specifically, U.S. Pat. No. 2,821,460 (Patent Literature 7) discloses a method of adding an aluminum chloride to chlorosilanes to form an AlCl3.PCl5 complex, followed by distillation and purification. Further, Japanese Patent Laid-Open No. 04-300206 (Patent Literature 8) discloses a method of adding an aqueous solution of inorganic salt such as TiCl4 in a high concentration to hydrolyze impurities to convert the impurities to a high-boiling-point compound, followed by distillation and purification.
There has been also proposed a method of adsorbing impurities contained in chlorosilanes to alumina, silica gel, activated carbon, or the like to thereby remove the impurities.
Specifically, U.S. Pat. No. 3,252,752 (Patent Literature 9) discloses a method of immobilizing a substance having a lone pair of electrons (for example, a substance such as propionitrile having a nitrogen atom and benzaldehyde having an oxygen atom) on an adsorbent such as activated carbon and silica gel and therethrough passing a chlorosilane gas to capture and remove impurities. Further, German Patent No. 1,289,834 (Patent Literature 10) discloses a method of bringing chlorosilanes in a state of liquid or vapor into contact with activated alumina to remove impurities. Furthermore, U.S. Pat. No. 4,112,057 (Patent Literature 11) discloses a method of bringing chlorosilanes into contact with metal oxides such as hydrated silica gel and alumina gel to remove impurities; and Japanese Patent Laid-Open No. 2001-2407 (Patent Literature 12) discloses a method of bringing chlorosilanes into contact with an alkali or alkaline earth fluoride salt to remove impurities.
Besides these methods, there has been proposed a method of obtaining chlorosilanes having a low impurity concentration by introducing a small amount of oxygen into chlorosilanes in a high temperature condition to react them with each other to form a complex; reacting the complex with donor impurities and acceptor impurities to form a new complex; and separating the new complex in a distillation step of chlorosilanes (refer to National Publication of International Patent Application No. 1983-500895 (Patent Literature 13)).